Many medical devices incorporate an elongated or tubular element that is required to be positioned at a particular anatomical site. Such devices include pacemakers, spinal cord and peripheral nerve stimulators for parathesia systems and functional electrical stimulation, and drug delivery catheters.
In the case of a pacemaker, for example, the leads may be threaded through a vein, and then anchored using a fixation element at the distal tip of the lead to reduce the risk of, or even prevent, dislodgement. Such a fixation element may be a tine, fin, or screw that is secured in the trabeculae or muscle tissue of the ventricle, atrium or cardiac vessel.
Sacral nerve stimulator leads may include a fixation element(s), such as a tine(s), projecting from the lead body to constrain movement of the lead body relative to the surrounding tissue. Tines on a sacral nerve lead, such as the InterStim™ lead available from Medtronic, Inc. of Fridley, Minn., generally are located at a substantial proximal distance from the electrodes and face in only one (proximal) direction. Such placement allows for relative movement of the electrodes as the muscle and connective tissue within which the tines are placed moves relative to the stimulation target.
A spinal cord stimulator (SCS) may include an implantable pulse generator (IPG) connected to one or more leads having one or more electrodes configured to deliver electrical energy to the spinal cord to block pain signals from reaching the brain. Small changes in electrode position may in some cases adversely impact the ability of such systems to effectively deliver therapy. It may not be practical or feasible to provide an anchoring mechanism inside the spinal canal to anchor a lead of the SCS. One conventional technique for securing the lead is to stabilize the lead using a ligature sleeve or suture sleeve secured to the lead body and attached to the superficial fascia with a suture as described, for example, in U.S. Pat. No. 5,957,968 to Belden and U.S. Pat. No. 7,930,039 to Olson. This technique, while commonly used, suffers from drawbacks including significant incidence of lead dislodgement. Another drawback is that the superficial tissue is often at an undesirable distance from the tissue targeted for stimulation. Any change in patient posture which results in a change in the relative distance between the superficial fascia and the stimulation target tissue may result in tension being applied to the lead body and subsequent movement of the electrodes.
U.S. Pat. Nos. 8,428,728 and 8,606,358 to Sachs and U.S. Patent Application Publication No. 2011/0224665 to Crosby et al., all assigned to the assignee of the present invention, and each of which is incorporated herein in their entireties by reference, describe implanted electrical stimulation devices that are designed to restore neural drive and rehabilitate the multifidus muscle to improve stability of the spine. Rather than masking pain signals while the patient's spinal stability potentially undergoes further deterioration, the stimulator systems described in those applications are designed to reactivate the motor control system and/or strengthen the muscles that stabilize the spinal column, which in turn is expected to reduce persistent or recurrent pain. Sachs and Crosby also describe in alternative embodiments peripheral nerve stimulation, in which electrical energy is applied to a nerve to effect a physiological change, such as to elicit a muscle contraction.
While the stimulator systems described in the Sachs patents and Crosby application seek to rehabilitate the multifidus and restore neural drive, use of those systems necessitates the implantation of one or more electrode leads in the vicinity of a predetermined anatomical site, such as the medial branch of the dorsal ramus of the spinal nerve to elicit contraction of the lumbar multifidus muscle. For that application, there is no convenient anatomical structure near the distal end of the lead to allow use of conventional anchoring mechanisms. Anchoring the lead to the superficial fascia as described above initially may be effective in many cases, but leads anchored in this manner may be susceptible to the problems of dislodgement and fatigue-induced fracture.
Previously-known efforts to overcome the problems of lead displacement abound. For example, U.S. Pat. No. 7,493,175 to Cates describes apparatus for subcutaneously anchoring a cardiac electrode lead using multiple tines. Such an apparatus would be undesirable for implantation in or adjacent to spinal muscle as the tines may become dislodged and tear the muscle during movement.
U.S. Pat. No. 7,797,053 to Atkinson describes a tether and a stent like device at the distal portion of a lead that may be expanded inside a cardiac vein to anchor a cardiac pacing lead. A similar stent-like anchor for a neurostimulation lead is described in U.S. Pat. No. 7,917,230 to Bly. U.S. Pat. No. 7,908,015 to Lazeroms describes a stimulation lead to be placed subcutaneously in which the fixation mechanism includes a movable mechanism at the distal end of the lead such that the lead diameter is increased at the distal end when engaged to provide anchoring. U.S. Pat. No. 8,170,690 to Morgan describes use of a helical element (screw) for anchoring a lead. These previously known anchoring systems are ill suited for neuromuscular stimulation because such systems have a high risk of dislodgement of the lead when implanted in or adjacent to muscle.
It therefore would be desirable to provide electrode leads and methods of implantation wherein the lead is securely anchored within a patient and is able to absorb axial movement and tensile load without distributing the load to the distal anchored end, thus reducing the risk of dislodgement of the lead and/or lead fracture.